Ordinarily, male zoo lions not intended for breeding have vasectomies, to preserve the mane and help to keep their “male” behaviors. Vasectomies can be reversed, too, which could be valuable in case a male may be needed for breeding in the future. It’s well known among zoos that castration usually causes loss of the mane. Maybe there was another reason for castrating this one. Excessive fighting with another male, maybe, or a medical problem.
Testosterone is not the entire answer, however; in some areas (notably Tsavo National Park in Kenya, but a few other places as well), MOST of the male lions have no manes, or if a mane is present, it’s very sparse. The lions of Tsavo have been said to be more aggressive than lions elsewhere, too, though it hasn’t been proven, There is ongoing field research to try to resolve the dilemma: if testosterone is responsible for the mane and the aggressiveness of male lions, why then do lions in some regions not have manes?
Another theory is that lions in cooler locations have manes, but lions in extremely hot, arid areas like Tsavo don’t grow them in response to the increased body temperature that a mane creates. But that doesn’t explain the apparent manelessness of European cave lions, which lived in temperate to very cold climates during the Pleistocene & thus would seem to have had manes, if temperature were the determining factor. It’s even been suggested that the Tsavo lions are a relict subspecies of cave lions (but genetic analysis doesn’t seem to support this).
There is ongoing field research to try to resolve these question. So far, the lions of Tsavo react to maned and unmaned decoy dummy lions exactly the same way as lions from the Serengeti and other areas where male lions generally have mane. One way to resolve the question of habitat vs. genetics would be to capture male cubs of lions from several areas and raise them together under identical conditions and document whether they grow manes and the rate of the manes’ growth. The logistics of such an experiment are daunting, though, and no one has yet attempted it.